The prime minister's former chief aide has offered to give evidence next month under oath and disclose emails and texts to a parliamentary inquiry into the handling of the Covid crisis.
In his blog post on Friday denying No 10's claims that he was the source of leaks to the media, Mr Cummings also referred to "WhatsApp messages with very senior officials".
Insiders have now told the Sunday Telegraph that they fear Mr Cummings could have a "treasure trove" of internal memos and emails which could damage the reputation of the government even further.
"Apparently he has got loads of stuff," said one source. "No one has a real clue about what documents, emails and texts he actually has. They don't know how much he has, nor what he has."
A second government source is said to have told the newspaper that Mr Cummings "used to scribble a lot in that book", adding: "I don't know what he has got, what he has not got."
Another said that those inside No10 were "s******* themselves".
According to the Telegraph, officials are concerned that the documents would reveal Mr Johnson and Rishi Sunak's strong opposition to the second lockdown.
Mr Cummings is due to give evidence to MPs on a joint parliamentary committee inquiry into the government's pandemic response on 26 May.
Jeremy Hunt, one of the joint chairs, told the Telegraph that they "will publish whatever he gives us" unless it raises national security issues.
In his blogpost Mr Cummings accused Boris Johnson of an "unethical, foolish, possibly illegal" plan to get Tory donors to secretly fund the lavish refurbishment of his Downing Street flat.
He denied that he was the source of leaks about the government's plans for a second lockdown, instead pointing the finger at Henry Newman, an advisor with close links to Cabinet Office secretary Michael Gove and Mr Johnson's fiancee Carrie Symonds.
Mr Cummings claimed that the prime minister tried to block the inquiry into the source of that leak for fear it would upset his girlfriend.
He also rejected claims he was the source of leaks about the texts between Mr Johnson and James Dyson, the UK's richest person, about tax issues.
On Saturday Scottish first minister Nicola Sturgeon said there was a "stench of sleaze" around the government and called for an inquiry into allegations of "contracts for cronies, donations for decorating and text messages for tax breaks".
Labour's shadow cabinet office minister Rachel Reeves has written to Mr Johnson demanding a "full investigation".
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